RectTransform的两个设置大小的api
var rtf = GetComponent<RectTransform>(); rtf.SetSizeWithCurrentAnchors(RectTransform.Axis.Horizontal, 200);
SetSizeWithCurrentAnchors不会改变anchorMin和anchorMax
设置大小会根据pivot的值，往两边扩充
var rtf = GetComponent<RectTransform>(); rtf.SetInsetAndSizeFromParentEdge(RectTransform.Edge.Left, 640-50, 200);
SetInsetAndSizeFromParentEdge会改变anchorMin和anchorMax
设置大小时，Edge为Left往右边扩充，Edge为Right往左边扩充，Edge为Top往下扩充，Edge为Bottom往上扩充
